1. Field of Invention
This invention pertains generally to engines and, more particularly, to a valve assembly and method for high temperature engines.
2. Related Art
Valves used in conventional Otto and Diesel engines function reasonably well for some load conditions but have limitations, particularly with advanced engine designs using separate combustion chambers as described and illustrated, for example, in U.S. Pat. Nos. 7,415,947 and 7,434,551.
In conventional engines with cam operated valves, the valve springs must be stiff or strong in order to close the valves quickly and without bounce at higher engine speeds. With such springs, the force or work required to open the valves significantly limits the efficiency of an engine. Strong, stiff springs also make electronic valving difficult because of the difficulty of generating electromagnetic forces that are strong enough to overcome them. Cam operated valves are also limited by the number of degrees of cam rotation required in order to provide even a moderate valve lift or opening, and this limitation may prevent cam operated valves from meeting the timing requirements of more advanced engines. Moreover, in conventional engines, valve cooling is often required in order to prevent heat damage to the valves, and such cooling further reduces engine efficiency.
With advanced engine designs, the valves must meet additional design and performance requirements to fully utilize the capabilities of an engine. In engines having separate combustion chambers, the valves must open into the combustion chambers where the pressure is typically on the order of 500 to 1000 PSI and the temperature is on the order of 1600° K to 1800° K. Since the valve assembly is located in the pressurized hot gas instead of the exhaust or intake manifold, valve actuation must be leak tight at high pressure and at high temperature. Moreover, because of the high temperature of the valve parts, the valve must be able to function without the use of traditional lubrication, and conventional valve guides probably cannot be employed as guides or bushings in the valve assembly. Furthermore, with variable valve timing and high speed operation in advanced engines, the valve opening and closing times must be short, and valve actuation strokes must also be short. Standard valves of the types heretofore provided cannot meet the demanding needs of advanced engines.